Angle or phase modulators are typically used in digital transmitters to encode messages in the phase of the output signal from the transmitter. High speed links require a large modulation bandwidth in the phase modulator. One method of modulation is to configure a single wide band loop so that all the modulation is performed inside the loop. In this manner, the loop stabilizes around the modulation. The main problem with using a single wide band loop is that a lot of noise is present. To address such a noise problem, a dual path modulation system can be used. This dual path modulation system typically keeps the main loop relatively narrow so that the noise can be controlled. However, using a narrow band main loop leaves fairly wide band modulations where the higher frequencies outside the main loop still have to be calibrated so that the overall gain is flat.
One dual-path method to is to use an angle or phase modulating system including a phase locked loop and apply the phase information to the main loop reference while simultaneously applying modulation directly to an analog voltage controlled oscillator used for the transmitter signal. The main loop reference is also called a direct path. The modulation is applied to a voltage controlled oscillator (VCO) via an auxiliary modulation path separate from the direct path. The variations in analog circuitry result in a mismatch between the phase indicated by the auxiliary modulation path and the phase indicated by the direct path. This variation must be calibrated.
FIG. 1 illustrates an exemplary block diagram of a conventional dual path angle modulator 10 as described in U.S. Pat. No. 6,094,101, which is incorporated herein by reference. The output of the main loop VCO 28 is mixed, via mixer 30, with the output of an offset PLL 32 to produce an IF (intermediate frequency) output signal frequency equal to the difference between the main loop VCO frequency and the offset loop frequency. The IF output signal is the feed back signal needed to measure the output phase and is processed in the remainder of the circuitry, which is all digital, to produce a control signal for the main loop VCO 28. The main loop VCO 28 produces the desired output signal phase. The digital circuitry has two paths, one low frequency path directly through the loop and a high frequency path with a scaling gain of MS. The scaling factor MS is adjusted for flat overall frequency response, and provides for balancing of the gain among the low-frequency, or direct, path and the high-frequency, or auxiliary, modulation path. Determination and application of the proper value of MS requires a calibration procedure.
During the calibration procedure, a known modulation signal is applied and the output is measured. Referring to FIG. 1, the known modulation signal is generated by the phase modulation generator 12, which is applied to the digital synthesizer 18 to output the signal S. An analog to digital converter (ADC) 34, such as a Sigma-Delta frequency to digital converter, provides a measured signal M which is a digital representation of the analog frequency output from the VCO 28. A logic circuit 36 receives the signal S and the signal M, and outputs an error signal Δ representing the frequency error between the signal S and the signal M. The error signal Δ is filtered using a digital filter, represented as the K1 block 20 and the K2/s block 22 in FIG. 1. The output of the K1 block 20 is a frequency error signal and the output of the K2/s block 22 is a phase error signal. The frequency error signal and the phase error signal are directed to a DAC 26 via a summing logic circuit 24. The scaling factor FS is used to add angle modulation waveform to the frequency error signal and the phase error signal via the summing logic circuit 24. The output signal of the DAC 26 is applied through resistors R1 and R2 to an integrating capacitor C1. The voltage stored on the integrating capacitor C1 is applied to the VCO 28.
The auxiliary modulation path is used to modify a modulation voltage applied to the VCO 28 in the main loop. The modulation signal generated by the phase modulation generator 12 is applied to a modulation DAC 42 via an MS multiplier 38. The MS multiplier 38 applies the scaling factor MS to the modulation signal. An output signal of the modulation DAC 42 is applied to the VCO 28 via the integrating capacitor C1. The modulating signal of the auxiliary path is scaled by the gain parameter MS, via the MS multiplier 38, and also scaled by the scaling factor FS, via the FS multiplier 40, and applied to the main loop at the summing logic circuit 24.
Achieving accurate wide bandwidth angle modulation in a phase locked loop is difficult. The method of using two or more paths to impress the phase information on the phase locked loop VCO is an established method. However, VCOs tend to drift and DACs also introduce inaccuracies. The gains in each of the two paths have to match. If the gain of the auxiliary modulation path (the path through which the higher frequencies pass) is too high, then too much modulation is applied to the VCO. If the gain of the auxiliary modulation path is too low, then there is insufficient modulation on the VCO. The means for balancing these two modulation paths has conventionally been achieved using a calibration procedure that focuses just on this calibration issue. The calibration procedure is either done manually using swept frequency techniques or done automatically using special calibration signals. However, such approaches are intrusive to the normal operation of such an angle or phase modulator because the calibration procedure can not be performed while the system is in operation. Instead, operation of the system must be suspended, and only during such a lapse time can the calibration procedure be performed.
Many conventional systems are designed to operate in bursts, which provide the necessary lapse time to perform the calibration procedure. Examples of such systems include GSM systems, other cellular networks, or any network using TDMA (time division multiple access). A transmitter included within such a system operates in a stand-by mode for a portion of the time. When it is time to transmit, the transmitter is awakened from stand-by mode, a calibration procedure is executed, the transmission is made, and the transmitter goes back to stand-by mode.
However, in newer generation systems, such as CDMA (code division multiple access), the transmitter is operating at all times, and there are no natural periods or intervals of down time where a calibration procedure can be performed. It is critical to the operation of a dual path modulator that both paths have exactly the same gain, so calibration of the two paths is still necessary.
In a conventional dual path angle modulator, if gains are wrong in the forward path of the control loop, then the entire signal is normalized within the control loop, including noise. A current practice is to make the bandwidth very wide so that the control loop automatically normalizes the entire wide band modulation signal. This practice yields a lot of noise in the output signal. As wider signal bandwidths are utilized, such as for wideband CDMA and wireless LAN, the resulting noise becomes impractical.